A ceramic honeycomb structure as a catalyst carrier in an exhaust gas purifying system for automobiles has a purpose of early activation of a carried catalyst. Therefore, the ceramic honeycomb structure is required to have a thin wall thickness of 140 μm or less, so that the heat capacity is decreased by total weight reduction.
Generally speaking, the ceramic honeycomb structure as the catalyst carrier receives outside pressure in a case. Since the thin wall thickness reduces damage resistance, there is a concern that the strength of the ceramic honeycomb structure against outside pressure will decrease. Especially, a hexagonal-cell honeycomb structure has a more serious problem of reduced destruction strength resistance than a triangle-cell or a square-cell.
Prior art is known that describes improving the strength of the honeycomb structure.
For example, Japanese Patent Laid-open Publication No. 54-110189 describes cell thickness of a structure regularly decreasing towards the center thereof.
As another example, Japanese Patent Laid-open Publication No. 10-264125 describes that a structure comprises a first area including a center of a structure and a second area located out side of the first area, and a reinforcement portion formed at each corner of the cells existing the second area.
In these prior art references, however, since there is a difference in cell wall thickness between an inside and an outside, the shrinking amount thereof is different in a sintering process.
As a consequence, there is a concern that an accurate dimension of the structure is not ensured and uniformity of the end surface of the structure is not ensured in an extrusion process.
Japanese Examined Patent Publication No. 3466862 describes a structure comprising a regular cell having a perfect shape and an irregular cell having a cross-sectional area less than 80% of the cross-sectional area of the regular cell and the thickness of only the irregular cells which are not perpendicular to an outer wall is greater than that of remaining cells by 9-100%.
Japanese Patent Laid-open Publication No. 11-277653 describes that a thickness “tr” of an outer cell partition, a thickness “tc” of a basic partition and a thickness “ts” of an outer wall have the relationship:0.7≦tc/tr≦0.9, 0.3≦tr/ts≦0.7
As above-mentioned, the prior art describes outer cell thickness thicker than inner cell thickness. Though these techniques can ensure the mechanical strength of the square-cell structure, they cannot ensure that of the hexagonal-cell structure.
Japanese Patent Laid-open Publication No. 2002-46117 describes that an intersectional radius (Rr) of an outer wall and a cell wall is larger than an intersectional radius (Rc) of each cell wall.
Japanese Patent Laid-open Publication No. 2003-269131 describes a structure comprising cells in which facing corners have a larger curvate corner than other facing ones.
Though these techniques can ensure the mechanical strength of the square-cell structure, these cannot ensure that of the hexagonal-cell structure.
As mentioned above, since these techniques relate to square-cells structures, even if these techniques are disposed in the hexagonal-cells structure, the mechanical strength of the hexagonal-cells structure may not be fully improved.